Redox-Active, Boron-Based Ligands in Iron Complexes with Inverted Hydride Reactivity in Dehydrogenation Catalysis

For
a series of PBP-type iron­(II) pincer complexes, the central
donor group based on tricoordinate boron is demonstrated to be redox-active,
formally yielding iron(0) and a boronium-species by reversible B–H
reductive elimination. In contrast to common tricoordinate boron compounds,
such as BF3, which are known to act as Lewis acid and Z-type
ligand, the introduction of π-accepting phosphine substituents
at the boron center leads to an umpolung of the bonding situation
from R3B←Fe to L2RB→Fe in the reported complexes. The described iron­(II)
complexes are competent catalysts for the dehydrogenation of Me2NH-BH3. Depending on the substituents a homo- or
hetertopic catalyst is formed. Experimental and quantum chemical investigations
on the most active, homogeneous catalyst indicate that hydrogen liberation
can proceed via different pathways, involving a hydrido ligand as
the proton source or a carbanion bifunctional mechanism. The unprecedented
catalytic mechanism and the unusual reactivity that allows for two-electron
redox steps are attributed to the unique donor properties of the boron-based
ligand.